Tuning into trace gases

New laser-based sensor detects scarce
compounds aloft

There’s a new instrument helping NCAR researchers in their
quest to quantify formaldehyde and other trace gases. Unlike some
of its predecessors, this instrument operates at room temperature
and travels well, even on a bumpy flight aboard a research aircraft.
The new tool, a product of NCAR’s Technology Development Facility
(TDF), promises to contribute to studies of atmospheric chemistry,
climate change, and a wide range of other areas.

If you install new carpet in your living room, there may be plenty
of formaldehyde floating around. In the free atmosphere, it’s
a different story. Formaldehyde (H2CO) enters the atmosphere as a
byproduct of combustion and industrial processes and is produced
in the atmosphere by the breakdown of hydrocarbons. Because it’s
so quickly oxidized, formaldehyde is one of the most scant of trace
gases, typically making up 1 to 50 parts per billion in
polluted urban areas and only about 10 to 100 parts per trillion
elsewhere. Yet formaldehyde is a major player in the formation of
ozone, a noxious and dangerous pollutant near ground level.

Trace gases are generally considered those that make up less than
1% of the atmosphere; for instance, carbon dioxide represents about
380 parts per million. Even this is plentiful compared to “ultra” trace
gases, which constitute still-tinier proportions of free air measured
in parts per billion or trillion. Researchers attempting to measure
ultra-trace-gas constituents have met with a host of difficulties
over the years. To be useful in field work, trace gas sensors must
be durable, sensitive, compact, selective, and affordable, none of
which are easy qualities to attain.

Of the lasers used to detect trace gases, small tunable lasers developed
for telecommunications are among the best. The catch is that the
gases that are most scarce tend to absorb most strongly in the mid-infrared
wavelength region, but most of the available laser sources operate
at shorter wavelengths. “To measure trace gases, you really
have to get into the strong mid-infrared absorption,” says
TDF director Alan Fried. “We have lasers that have gotten us
there, but they have to be cooled by liquid nitrogen.” This
process adds a lot of weight to airborne payloads, and the laser
source and associated optical system are not as compact, reliable,
and readily available as scientists would prefer.

The new NCAR sensor relies on a technology called difference frequency
generation (DFG). It’s being used at only a handful of labs
around the world, mostly for fundamental lab studies rather than
field work. Dirk Richter became familiar with DFG while completing
his doctorate at Rice University under Frank Tittel. After Richter
joined NCAR in 2000, he and Fried began paving the way for the first
use of DFG in atmospheric science. They were joined on the project
by Petter Weibring, who came to NCAR as a postdoctoral researcher
in 2003 with a decade-plus of lidar and instrumentation experience,
and by associate scientist Jim Walega, who has worked with instrumentation
for field campaigns for more than two decades.

A useful blend

Like a painter who mixes two common dyes to simulate a rare third
color, the DFG process takes two near-IR laser beams and mixes them
in a crystal to produce the difference frequency. The result is a
narrow beam of light in the desired mid-IR wavelength, generated
without the bulky setup normally needed. “The lasers we use
are common, off-the-shelf types,” says Fried.

Another advantage of the tunable DFG lasers is that they can be adjusted
to produce a variety of mid-IR wavelengths. “You can have
unique mixing schemes,” says Richter. This opens the door for
measuring other extremely scant gases. One example is 13CO2, an isotope
of carbon dioxide found in a small fraction of carbon dioxide molecules,
most of which are 12CO2. Because 13CO2 is taken up by vegetation
more slowly than standard 12CO2, “measuring the 13CO2-to-12CO2
ratio is an important new tool for studying dynamics of the carbon
cycle,” says Fried.

The acid test for the DFG device was whether it would hold up under
the rigors of airborne research. Early last year, the TDF group took
the new instrument on its maiden voyage aboard the NSF/NCAR C-130
during the Megacities Impacts on Regional and Global Environment
(MIRAGE) study in Mexico. Despite a rough start, says Walega, the
team made modifications and adjustments and the kinks were quickly
worked out.

Further improvements were made between the next two campaigns (a
NASA follow-up to MIRAGE and NOAA’s Texas Air Quality Study
of 2006), so that, in the end, the instrument worked semi-autonomously.
By the last mission, says Richter, “the technicians were afraid
the instrument wasn’t working because none of us were tweaking
it anymore.” He adds that these three campaigns marked the
first time in the world a DFG system was operated on an airborne
platform.

If funding comes through, plans are in the works to deploy the new ­instrument
in 2008 aboard NASA’s DC-8 aircraft for the agency’s
Arctic Research of the Composition of the Troposphere from Aircraft
and Satellites program. Along with detecting formaldehyde, the TDF
team may also try to measure methanol. “The ultimate goal
is to develop a multi-species sensor that can be run autonomously
on the NSF/NCAR Gulfstream-V aircraft,” says Weibring.

Above is a schematic of the
difference frequency generation technique as applied
at NCAR. An input signal with a wavelength of 1.5 microns
(red, at left) and a 1-micron “pump” signal
(blue, at left) are fed through a lithium niobate crystal.
The output includes the original input and pump signals,
as well as an “idler” signal of 3.5 microns—the
so-called difference frequency generated in the crystal.
Normally it would take a far more cumbersome operation
to produce the 3.5-micron signal.

The 22 January edition of the journal Optical Express includes a
report written by Richter, Weibring, and Fried. They’re joined
on the paper by four colleagues from NTT Photonics Laboratories,
the Japanese maker of a new type of
lithium niobate crystal used to route and combine the laser beams. “They
read our papers and felt we were one of the best groups to push the
technology and see how far we could go,” says Richter. A common
ingredient in cell phones, these new lithium niobate crystals enabled
the NCAR researchers to make the DFG process roughly a hundred times
more efficient by squeezing the incoming light into a fiber-like
structure only six microns wide. As Richter notes, “It’s
all very tiny stuff.”